Sprint is pretty atrocious. I had to use the Nextel part of there network for years at work and it was just sad. Sprint's only reason for being in business is the price advantage, which isn't what it used to be.
You'll never regret going to Verizon. However, AT&T gets a far worse wrap than they deserve. They've really expanded their coverage significantly over the last 18 months, and they are much faster than Verizon in most markets with data. My fellow SysAdmin where I work has a work Verizon droid and a personal iPhone 3GS on AT&T and he consistently has nearly twice the 3G speed and usually better coverage. This is in the Greater Hampton Roads, VA.
Of course, up to 6 or so months ago, he was cursing AT&T, so I think they did a big rollout here not to long ago.